Efficient generation of quantum-correlated or entangled photon pairs is of paramount importance for several applications of quantum communication and quantum metrology. In particular, quantum cryptography profits significantly when the schemes exploiting entangled photons are applied. For the practical implementation of cryptographic schemes, the polarization encoding of entangled-photon pairs showed to be the best choice mainly due to availability of simple and efficient polarization-control elements and analyzers. The source suitable for the realization of the industrial prototypes of entanglement-based quantum cryptography or possibly other applications of quantum communication has to have a high performance and furthermore has to fulfill several additional requirements regarding compactness, simplicity and robustness of design, as well as low operation costs or “push-button” operation requiring minimum alignment.
Since the first demonstration of a source producing polarization-entangled photon pairs based on an atomic decay in the beginning of the 80's, a wealth of theoretical proposals and experimental prototypes have been reported. To date, the most effective way how to produce entangled photon pairs showed to be via second-order (X(2)) nonlinear processes in crystals and third-order (X(3)) nonlinear processes in fibers. Due to very advanced stage of possible applications, such as quantum cryptography, the present development of the sources focuses largely on their simplification and miniaturization while keeping their performance on a high level. The performance of sources can be quantified in terms of their brightness (and spectral brightness), measured conveniently as number of detected pairs per second and milliwatt of pump power (and per nanometer of spectral bandwidth) and the achieved quantum-interference visibility of polarization-entanglement.
The background information together with the state-of-the-art relevant for the present invention is documented in the following patents and other publications:    [1] New high-intensity source of polarization-entangled photon pairs, P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Y. H. Shih, Phys. Rev. Lett. 75, 4337 (1995).    [2] High-efficiency entangled photon pair collection in type-II parametric fluorescence, C. Kurtsiefer, M. Oberparleiter, H. Weinfurter, Phys. Rev. A 64, 023802 (2001).    [3] U.S. Pat. No. 6,424,665 Ultra-bright source of polarization-entangled photons, P. G. Kwiat, P. H. Eberhard, A. G. White.    [4] Ultra-bright source of polarization-entangled photons, P. G. Kwiat, E. Waks, A. G. White, I. Appelbaum, P. H. Eberhard, Phys. Rev. A 60, R773 (1999).    [5] U.S. Pat. No. 6,982,822 High-flux entangled photon generation via parametric processes in a laser cavity, M. C. Teich, B. E. A. Saleh, A. V. Sercienko, J. T. Fourkas, R. Wolleschensky, M. Kempe, M. C. Booth.    [6] Cavity-enhanced generation of polarization-entangled photon pairs, M. Oberparleiter, H. Weinfurter, Opt. Commun. 183, 133 (2000).    [7] Phase-compensated ultra-bright source of entangled photons, J. B. Altepeter, E. R. Jeffrey, P. G. Kwiat, Opt. Express 13, 8951 (2005).    [8] EP 1 477 843 A1 Entanglement photon pair generator, S. Takeuchi.    [9] Source of polarization entanglement in a single periodically poled KTiOPO4 crystal with overlapping emission cones, M. Fiorentino, C. E. Kuklewicz, and F. N. C. Wong, Opt Express 13, 127 (2005).    [10] Theory and experiment of entanglement in a quasi-phase-matched two-crystal source, D. Ljunggren, M. Tengner, P. Marsden, and M. Pelton, Phys. Rev. A 73, 032326 (2006).    [11] Phase-stable source of polarization-entangled photons using a polarization Sagnac interferometer T. Kim, M. Fiorentino, F. N. C. Wong, Phys. Rev A 73, 012316 (2006).    [12] U.S. Pat. No. 6,897,434 All-fiber photon-pair source for quantum communications P. Kumar, M. Fiorentino, P. L. Voss, and J. E. Sharping.
[13] Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, Phys. Rev. Lett. 94, 053601 (2005).
[14] Photon pair generation using four-wave mixing in a microstructured fibre: theory versus experiment, O. Alibart, J. Fulconis, G. K. L. Wong, S. G. Murdoch, W. J. Wadsworth, J. G. Rarity, N. J. Phys. 8, 67 (2006).
[15] JP2005257941 Entangled photon pair generating device and its method, I. Yasushi.
[16] WO2005103810 Method for generating quantum-entangled photon pair K. Edamatsu, T. Itoh.
[17] A semiconductor source of triggered entangled photon pairs, R. M. Stevenson, R. J. Young, P. Atkinson, K. Cooper, D. A. Ritchie, A. J. Shields, Nature 439, 179 (2006).
To date, the simplest and the most established way how to generate entangled photon pairs is parametric fluorescence (spontaneous parametric down-conversion, SPDC) in second-order (X(2)) nonlinear materials. In this process photons of an intense pump beam spontaneously convert in a nonlinear optical crystal with a low probability into two daughter photons. Energy and momentum conservation ensures that the emitted photons exhibit nonclassical correlations in these two continuous degrees of freedom. Via [1-4] two basic methods are known how to obtain polarization-entanglement from SPDC. The first uses type-II phase-matching (i.e. the emitted photons are mutually orthogonally polarized) in non-collinear geometry (i.e. generated fluorescence photons propagate in directions not identical with the propagation direction of pump photon) and the second makes use of two crossed type-I phase-matched nonlinear crystals (i.e. two identically polarized photons can be produced in either crystal) in non-collinear geometry. These sources are inherently wide band, generally low brightness and require careful alignment.
Various variations of the original proposals were published over the past few years. References [5] and [6] present the method to increase the intensity of pump field for SPDC by confinement the nonlinear crystal in a cavity. Reference [7] shows that the use of special birefringent compensation elements can increase the collected usable solid angle of the down-conversion light. Patent publication [8] disclose the method how to converge the down-conversion light extensively generated in a wide solid angle in a beam shape leading to a higher coupling efficiency into single-mode fibres. Particularly promising, as reported in [9-11], show to be the methods, where conventional nonlinear crystals are exchanged by quasi-phase matched periodically poled crystals. This enables the access to higher nonlinear coefficients and thus better conversion efficiency of pump photons into down-conversion photons. In general, all the aforementioned variations enhanced the performance of the sources, particularly regarding their brightness, however often at the expense of their bigger complexity, increased costs and higher demand for alignment.
A great drawback, limiting the performance and flexibility of all aforementioned sources is that two separate spatial modes defined by coupling optics, irises or single-mode fibres are used for collecting fluorescence photons. High-purity polarization entanglement can be obtained only if the divergences of these modes and also their mutual spatial orientations are perfectly matched. Consequently, a careful alignment of all the optics defining collection modes, is always required.
With respect to brightness and integration into communication fibre networks, promising sources of correlated photon pairs were disclosed in publications [12-14]. There, the correlated photon pairs at unequal wavelengths were generated via four-wave mixing process in optical fibres with nonzero third-order (X(3)) nonlinearity. The polarization-entangled photon pairs can be obtained using counter propagating pumping of a straight fibre or a fiber Sagnac loop, in accordance with [15]. Due to significant Raman background requiring careful filtering of correlated photons, and particularly, the need of mode-locked picosecond lasers for pumping four-wave mixing process, the sources are complex and financially demanding.
Other currently pursued way how to generate entangled photon pairs, namely decay processes in quantum dots or other semiconductor structures [16, 17], were not successful, and no entanglement or entanglement of a poor quality was observed till now.
It is an object of the present invention to provide an improved apparatus for the generation of polarization-entangled photon pairs and method for such generation, overcoming the deficiencies of the prior art, including those outlined above.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.